1. Field of the Invention
The present invention relates to a polymeric substance and an additive for the screening of solar radiation with the aim of influencing plant growth and the morphogenesis.
2. Brief Description of the Related Art
It is necessary for a controllable growth of plants to set up special light conditions to affect plant growth and morphogenesis. Special light conditions for plant growth, for example, within a greenhouse or under a mulch film include some which are of particular interest. The temperature within the enclosed area and the light intensity and the spectral distribution of the incoming light are important factors for the plant growth.
Plants, grown in a greenhouse build a resistance to the variations in temperature. From this it follows that the plants stay back in growth. Thus, for the photosynthesis in greenhouses the photosynthetically active radiation (PAR) is only desired while other parts of the solar light should be screened because they have mainly a negative influence on the microclimate. Therefore the following objectives have to be achieved. Firstly, the intensive irradiation by sunlight in particular by NIR radiation needs to be screened, in order to avoid too high temperatures within the enclosed area. Secondly, the incoming light beams are to be scattered in order to prevent burnings on plants. Thirdly, heat within the area is to be stored at night.
However, in case of mulch films the opposite effect is desired. The PAR should be screened and all other parts of the solar light should be transmitted. Mulch films are used in agriculture to improve the growing conditions of the useful plants and to minimize the use of chemical herbicides.
For certain crop growers want to intervene in the plant morphogenesis. Morphogenesis is the influence from environmental factors on the shape and appearance of the plant. One of them is the light quality. It means that the spectral energy distribution has an effect on the growth of plants.
Raviv and Allingham describe in xe2x80x9cPlastic Culturexe2x80x9d, No. 59, September 1989, pp. 3 to 12, that diffuse light is favourable for a steady growth of plants. Furthermore, it turned out that diffuse light has the advantage that the plants are not going to be damaged. Hancock describes in xe2x80x9cPlastic Culturexe2x80x9d, No. 79, 1988, pp. 4 to 14, that special additives in LDPE have a favourable effect on the microclimate in greenhouses because of the generation of diffuse light and the creation of a favourably thermic effect in the greenhouses. Suitable additives are aluminosilicate in the form of kaolin, calcium carbonate, talc and kaolin clay.
Greenhouses made of glass already fulfil some of the above-mentioned conditions by virtue of the inherent physical properties of inorganic glasses. Furthermore, constructive measures, for example knurled glass or shadow cloth are known.
If the enclosing material is plastic, such as, for example, polyethylene, polyethylene/vinyl acetate copolymers, polyvinyl chloride, polycarbonate or polyacrylate, it is much more difficult to meet these requirements. The high transparency of many plastics for IR radiation results in good heat transmission which, in the case of films, is additionally assisted by the low film thickness.
A polymethyl methacrylate glazing material for buildings and vehicles containing an interference pigment for the screening of NIR radiation of a wave length between 800 and 1500 nm is disclosed in DE 2,544,245.
The pigment used has a blue-red colour and the light transmitted through the glazing material has a yellow-green colour. When used in greenhouses, this glazing material has the disadvantage that portions of visible light which cannot be utilized by the plant are transmitted and, on the other hand, the red portion of visible light which can be utilized by the plant is screened off by said glazing material. Furthermore, the transmitted green portion has the additional disadvantage that it contributes to heating of the greenhouse by virtue of its conversion into long wave light.
EP-A 0,428,937 describes a grey-white coating composition for greenhouses consisting of a polymer substrate and reflecting particles suspended therein. These particles are aluminium platelets or mica platelets coated with titanium dioxide. The coating composition is used for temporary coatings in extreme weather conditions (summer). In winter, it can again be removed by washing it off with a water jet. More details regarding the pigment used are not given.
The coating composition has the disadvantage that not only the IR beams but also a substantial portion of the visible light which can be utilized by the plant are reflected by the metal particles. The green portion of the transmitted light cannot be utilized by the plant and contributes to heating of the greenhouse.
EP-A-03 98 243 describes polymeric mulch sheets and mulch films for use in agriculture which have a green absorption colour and contain a UV stabilizer. The green sheets and films absorb a large part of the solar radiation which promotes photosynthesis and plant development (PAR), and transmits enough solar radiation so as to heat the soil beneath such films and sheets. Such mulch films and sheets reduce weed growth but they have the disadvantage that the photosynthetically active radiation is absorbed and is lost for the useful plant.
L. M. Mortensen and E. Stromme describe in Scientia Horticulturae 33 (1987), 27-36 the effects of light quality on some greenhouse crops. Different light qualities were established in growth chambers placed outdoors by selective screening of the natural light spectrum in the chambers. The PAR level was the same at all light qualities. Blue light containing a high red/far-red ratio reduced the dry weight compared to natural (N), green, yellow and red light in chrysanthemum, tomato and lettuce. Plant height in chrysanthemum and tomato was strongly reduced by blue compared to N light, and strongly increased by green and yellow light.
The aim of the invention is.,to provide a composite material for the screening of intensive irradiation by sunlight, which material reflects, apart from the IR portion, also a green portion of visible light which cannot be utilized by the plant.
A further aim of the invention is to provide a composite material useful as mulch film or mulch sheet which do not absorb the photosynthetically active radiation.
Lastly, it is the aim of the invention to provide a composite material useful as covering material for the influencing of plant morphogenesis.
This object is achieved according to the invention by a composite material comprising a polymer and an additive and the customary stabilizers and processing aids necessary for the particular polymer.
The additive according to the invention is a platelet-shaped material or a platelet-shaped material coated with one or more metal oxides.
Usually the platelet-shaped material consists of layered silicate, synthetic mica, glass platelets, ceramic platelets and silicia platelets.
Preferably the layered silicate is mica, pyrophillite, sericite, talc or kaolin.
The platelet-shaped material coated with one or more metal oxides is preferably an interference pigment.
The preferably used interference pigments are platelet-like pigments consisting of platelet-like substrate, such as for example, mica, kaoline or talc, and one or more metal oxide films thereon.
The metal oxides which can be used are only those oxides which allow the preparation of interference pigments, for example, oxides of the metals tin, titanium, zirconium, chromium, cerium, iron or tungsten.
In order to be able to produce interference colours, a certain film thickness is necessary. This film thickness depends on the metal oxide used. If titanium dioxide is used, it is for example advantageous to apply a film in a thickness of 120 to 160 nm for obtaining a pigment having a green interference colour.
In the invention the substrate preferably has a diameter of from 1 to 200 Am and more preferably from 5 to 60 xcexcm and a thickness of from 0.1 to 10 xcexcm, preferably from, 0.5 to 1.5 xcexcm.
The additive content of the composite material is 0.1 to 30% by weight, preferably 0.5 to 5% by weight, based on the polymer. The additive content depends on the film thickness of the composite material. The term xe2x80x9cadditivexe2x80x9d includes also mixtures of additives.
For the composite materials according to the invention, it is possible to use as additive commercially available interference pigments, which are available under the trade name IriodinR manufactured by E.MERCK, Darmstadt. They consist of mica, TiO2 and SnO2.
The polymer according to the invention may be an organic or inorganic polymer.
Said organic polymers used for the preparation of the composite materials are transparent materials like polyolefins, co- or terpolymers, for example low-density polyethylene (LDPE), vinylester copolymers like ethylene-vinylacetate copolymer (EVA), fluoropolymers, co- or terpolymers like polytetrafluoroethylene (PTFE) and polyvinylidene chloride (PVDC), polyvinyl chloride (PVC), polycarbonate (PC), polymethylacrylate (PMMA) or mixtures of transparent polymers. The term xe2x80x9cpolymerxe2x80x9d may include mixtures of polymers.
As inorganic polymer glass may be used.
The composite materials additionally may contain the stabilizers and processing aids necessary for the particular polymer.
The composite material according to the invention can be used in the form of films, sheets, spinbonded and woven fabrics and profiles, preferably in plant cultivation and in plant growing (agriculture and gardening). The composite material according to the present invention can be used, for example, in the form of films or plates for greenhouses, as tunnel films and mulch films. Furthermore, it can be used in the form of film for covering of soils or as substrate film. It can be produced using the customary plastic processes, such as film blowing, film casting, coextrusion, cast film extrusion or lamination in the form of films, multilayer films or sheets.
For optimum effect of the interference pigment, it is necessary to align the pigment platelets in the composite material parallel to the surface and parallel to one another.
This must be taken into account, for example, in casting. In other processing procedures, this parallel alignment necessarily follows from the flow characteristic of the melt.
For the effectiveness of the pigment, it is immaterial whether the polymer is completely coloured or the pigment is present in one or more layers of a multilayer system. Such multilayer systems can be produced by coextrusion, coating, extrusion coating, laminating or printing.
The composite material according to the invention used as covering for greenhouses preferably contains a green interference pigment. That means that the longwave infrared radiation is absorbed, the near infrared radiation is reflected together with the green portion of the incoming visible light and only the red and blue portions of visible light which can be utilized by the plant are transmitted and scattered.
On the other hand the composite material absorbs the infrared radiation which is reflected or emitted by the ground or the contents of the enclosed areas and prevents a too strong cooling of the enclosed areas at night. Consequently, the incoming light is used optimally for the climate under the composite material.
In case the composite material according to the invention is used as mulch film or mulch sheet it is desired to screen the photosynthetically active radiation in order to prevent the growth of weeds. Therefore, according to the invention an additive, preferably an interference pigment is used, which reflects the PAR. The reflection of the PAR by the mulch film or sheet has the advantage that the useful plant which grows above the mulch film can use the reflected PAR for its growth. In case of a black film or a green film coloured with an absorption pigment the PAR is absorbed and is lost for the useful plant.
According to the invention a red-blue interference pigment, for example IRIODINR 219 Rutile Lilac Pearl, is used. This mulch film or mulch sheet can be used alone or in combination with a second layer or a separate film containing known absorption pigments below. This could be done, for example, in a two or multilayer film in which the interference pigment is in the upper layer while the absorption pigment is in the lower layer.
As a result the mulch films or mulch sheets according to the invention improve the microclimate below the film (temperature and humidity of the soil) and in addition to known films improve the growth of the useful plants by reflecting the PAR.
When the composite material according to the invention is used for modifying the plant morphogenesis an additive, preferably an interference pigment is used which changes the spectral distribution of the sunlight.
For certain crops growers want to modify the plant morphogenesis. Morphogenesis is the influence from environmental factors on the shape and appearance of the plant.
It is well known in agriculture that growth, blooming and crops can be influenced by the spectral composition of light.
Plants contain pigments which can deduct of the incoming light, certain wave lengths and their intensity. The most important plant pigments for morphogenesis are the phytochrome and cryptochrome. Phytochrome absorbs mainly in the Red/Far Red region (680 nm/730 nm). Cryptochrome absorbs in the blue and UV-A region (315 to 500 nm).
Photoselective light in the Blue or in the Red/Far Red region activates the plant pigments positively or negatively and plays an important role in growth and flowering, for example.
These relations were found by agricultural experimentsxe2x80x94in which different lamps, filters and coloured films were used. The use of filters and coloured films always has the disadvantage of a loss of energy which reduces the efficiency.
The present invention describes a novel method to influence the photogenesis by using interference pigments. Light passing through the composite material containing interference pigments can influence the physiological responses of plants. This is due to changes in the Red/Far Red ratio, Blue/ Red ratio and Blue/Far Red ratio which control plant morphology. Reducing Red/Far Red ratio with a blue interference pigment like IriodinR 225 Rutile Blue Pearl or IriodinR 221 Rutile Fine Blue results in plants with longer internodia, fewer branches and longer but smaller leaves.
Bolting can be reduced by increasing the Blue/Red ratio (relatively richer in Blue than in Red) using a composite material which contains a yellow interference pigment like IriodinR 205 Rutile Platinum Gold.
When a more compact plant growth (suppressed stem elongation) is required a high Red/Far Red ratio combined with a high Blue/Red ratio is obtained by a composite material containing a yellow-green interference pigment of second order. In these cases the use of hormonal growth regulators could be avoided.
Depending on the plants and the aim, a suitable interference colour can be chosen of the wide range of colours. The maximum reflection (minimum transmission) can be brought to every wave length by creating a layer thickness of the interference pigments. The selection has to be done according to the state of the knowledge. The use of interference pigments gives the possibility to select a certain spectrum without the loss of too much radiation energy.
As a consequence of the transparency and the absence of absorption in the interference pigments used, the incoming light is hardly diminished by scattering and absorption. Measurements have shown that only about 5% of the increasing light is absorbed. The low heat conductivity of the pigment used has a positive effect on the thermal resistance and the heat storing capacity of the composite materials according to the invention. The selective reflection results in a uniform climate within the greenhouse in almost any weather without screening off, to any great extent, the portions of visible light which can be utilized by the plant.